Zero Profile Spinal Fusion Cage

ABSTRACT

An interbody fusion cage having upper and lower canals for receiving the heads of bone screws that have been pre-installed in opposing vertebral body endplates. The proximal wall of the cage preferably has a vertical slot that communicates with each canal and is adapted to allow access by a screwdriver and tightening of the screws.

CROSS-REFERENCE TO RELATED APPLICATIONS

This is a continuation application of U.S. patent application Ser. No.12/414,532 filed Mar. 30, 2009, the disclosure of which is herebyincorporated by reference as if set forth in its entirety herein.

BACKGROUND OF THE INVENTION

The natural intervertebral disc contains a jelly-like nucleus pulposussurrounded by a fibrous annulus fibrosus. Under an axial load, thenucleus pulposus compresses and radially transfers that load to theannulus fibrosus. The laminated nature of the annulus fibrosus providesit with a high tensile strength and so allows it to expand radially inresponse to this transferred load.

In a healthy intervertebral disc, cells within the nucleus pulposusproduce an extracellular matrix (ECM) containing a high percentage ofproteoglycans. These proteoglycans contain sulfated functional groupsthat retain water, thereby providing the nucleus pulposus within itscushioning qualities. These nucleus pulposus cells may also secretesmall amounts of cytokines such as interleukin-1β and TNF-α as well asmatrix metalloproteinases (“MMPs”). These cytokines and MMPs helpregulate the metabolism of the nucleus pulposus cells.

In some instances of disc degeneration disease (DDD), gradualdegeneration of the intervertebral disc is caused by mechanicalinstabilities in other portions of the spine. In these instances,increased loads and pressures on the nucleus pulposus cause the cellswithin the disc (or invading macrophages) to emit larger than normalamounts of the above-mentioned cytokines. In other instances of DDD,genetic factors or apoptosis can also cause the cells within the nucleuspulposus to emit toxic amounts of these cytokines and MMPs. In someinstances, the pumping action of the disc may malfunction (due to, forexample, a decrease in the proteoglycan concentration within the nucleuspulposus), thereby retarding the flow of nutrients into the disc as wellas the flow of waste products out of the disc. This reduced capacity toeliminate waste may result in the accumulation of high levels of toxinsthat may cause nerve irritation and pain.

As DDD progresses, toxic levels of the cytokines and MMPs present in thenucleus pulposus begin to degrade the extracellular matrix, inparticular, the MMPs (as mediated by the cytokines) begin cleaving thewater-retaining portions of the proteoglycans, thereby reducing itswater-retaining capabilities. This degradation leads to a less flexiblenucleus pulposus, and so changes the loading pattern within the disc,thereby possibly causing delamination of the annulus fibrosus. Thesechanges cause more mechanical instability, thereby causing the cells toemit even more cytokines, thereby upregulating MMPs. As this destructivecascade continues and DDD further progresses, the disc begins to bulge(“a herniated disc”), and then ultimately ruptures, causing the nucleuspulposus to contact the spinal cord and produce pain.

One proposed method of managing these problems is to remove theproblematic disc and replace it with a porous device that restores discheight and allows for bone growth therethrough for the fusion of theadjacent vertebrae. These devices are commonly called “fusion devices”or “fusion cages”.

Current interbody fusion techniques typically include not only aninterbody fusion cage, but also supplemental fixation hardware such asfixation screws. This hardware adds to the time, cost, and complexity ofthe procedure. It also can result in tissue irritation when the cage'sprofile extends out of the disc space, thereby causingdysphonia/dysphagia in the cervical spine and vessel erosion in thelumbar spine. In addition, the fixation hardware typically includes asecondary locking feature, which adds to the bulkiness of the implantand time required for the procedure. Furthermore, existing fixationhardware may prevent the implantation of additional hardware at anadjacent location, and so require removal and potentially extensiverevision of a previous procedure.

US Published Patent Application 2008-0312698 (Bergeron) discloses adevice and system for stabilizing movement between two or more vertebralbodies and methods for implanting. Specifically, the embodiments providemedical professionals with the ability to selectively position andorient anchors in bony tissue and then attach a plate to thepre-positioned anchors. The plate assembly, once positioned on theanchors, prevents the anchors from backing out of the bony tissue.Furthermore, in situations in which it is desirable to provide spacingbetween two vertebral bodies, a spacer may be fixedly connected to theplates for positioning between two vertebral bodies. The spacer mayfurther function as a lock out mechanism, or may be rotatably connectedto the plates to maintain rotational freedom. The spacer may incorporateconnection features or attachment features.

U.S. Pat. No. 4,904,261 (Dove) discloses a spinal implant, e.g., toreplace an excised disc, comprising a rigid generally horseshoe shape ofbiocompatible material, such as carbon-fibre reinforced plastics, havingupper and lower planar faces converging towards the ends of thehorseshoe, and at least one hole from each planar face emerging in theouter curved face of the horseshoe, to enable the horseshoe to be fixedby screws inserted through one or more selected holes in each pluralityfrom the ends in the outer curved face into respective adjacentvertebrae, with the screw heads bearing against shoulders, and with thespace bounded by the inner curved face of the horseshoe available forthe insertion of bone graft or a bone graft substitute.

U.S. Pat. No. 6,579,290 (Hardcastle) discloses a surgical implant forfusing adjacent vertebrae together comprising a body portion with spacedarms. The body portion has passages to receive surgical fixing screwsengaged in holes drilled in the vertebrae for securing the body portionto the anterior faces of the vertebrae to be fused. The arms extend intoa prepared space between the vertebrae to be fused. Graft material ispacked between the arms. Each surgical fixing screw has an externallyscrew-threaded shank divided into wings which can be outwardly deformedto anchor the shank in the hole. Each surgical fixing screw also has ahead which can be transformed between a laterally expanded condition anda laterally contracted condition to permit the head to be interlockedwith the implant.

U.S. Pat. No. 6,342,074 (Simpson) discloses a spinal fusion implant andmethod for maintaining proper lumbar spine curvature and intervertebraldisc spacing where a degenerative disc has been removed. The one-pieceimplant comprises a hollow body having an access passage for insertionof bone graft material into the intervertebral space after the implanthas been affixed to adjacent vertebrae. The implant provides a pair ofscrew-receiving passages that are oppositely inclined relative to acentral plane. In one embodiment, the screw-receiving passages enablethe head of an orthopedic screw to be retained entirely within theaccess passage. A spinal fusion implant embodied in the presentinvention may be inserted anteriorly or laterally.

U.S. Pat. No. 6,972,019 (Michelson) discloses a spinal fusion implantfor insertion between adjacent vertebral bodies that has opposed upperand lower surfaces adapted to contact each of the adjacent vertebralbodies from within the disc space, a leading end for insertion betweenthe adjacent vertebral bodies, and a trailing end opposite the leadingend. The trailing end has an exterior surface and an outer perimeterwith an upper edge and a lower edge adapted to be oriented toward theadjacent vertebral bodies, respectively, and a plurality of bone screwreceiving holes. At least one of the bone screw receiving holes isadapted to only partially circumferentially surround a trailing end of abone screw received therein. At least one of the bone screw receivingholes passes through the exterior surface and one of the edges so as topermit the trailing end of the bone screw to protrude beyond one of theedges.

US Patent Publication 2009-0030520 (Biedermann) discloses a fixationdevice for bones that includes a member which is to be fixed to one ormore bones and has at least one bore for receiving a bone screw, whereinthe at least one bore comprises a first internal thread portion. Thebone screw has a first shaft section provided with a first externalthread portion arranged to cooperate with the internal thread portion ofthe at least one bore, and a head section having a diameter larger thanthat of the shaft section to provide a catch arranged to engage with astop formed in the bore. The bone screw further has a second shaftsection which includes a clearance groove extending between the catch ofthe head section and the external thread of the first shaft section. Theclearance groove allows disengagement of the two thread portions, suchthat the bone screw is prevented from being unscrewed off the bore whenit is loosened within the adjacent bone. The member can also include aside wall of a cage used in an intervertebral implant device, or canrepresent a plate of a bone plate assembly.

SUMMARY OF THE INVENTION

The present invention is directed to a method of fixing anintervertebral fusion cage in a disc space. In this method, a pair offixation screws are first inserted into the opposing vertebral endplateswithin the disc space so that only their heads are exposed. These screwheads do not extend out of the disc space. Next, a novel cage (which hasupper and lower longitudinal depressions that act as screw guidesurfaces) is slid into the disc space using the screw heads as guides.When the cage is fully inserted, each screw head becomes seated in adistal (preferably, deeper) portion of the depression located in theproximal portion of the cage, thereby locking the cage in place. In thisfixed condition, both the cage and the screw heads are located fullywithin the disc space and thereby provide a zero-profile assembly.

Therefore, in accordance with the present invention, there is providedan intervertebral fusion cage comprising:

-   -   a) a proximal wall and a distal wall;    -   b) first and second side walls connecting the proximal and        distal walls, an upper bearing surface adapted for gripping an        upper vertebral endplate and a lower bearing surface adapted for        gripping a lower vertebral endplate, the upper bearing surface        having at least one upper opening therethrough adapted to        promote bony fusion, the lower bearing surface having at least        one lower opening therethrough adapted to promote bony fusion,        and    -   c) a first guide surface canal formed in the upper bearing        surface and extending substantially from the proximal wall to        the distal wall, the canal adapted for distal reception of a        first screw head.

Also in accordance with the present invention, there is provided anintervertebral fusion cage, comprising:

-   -   a) a proximal wall and a distal wall,    -   b) first and second side walls connecting the proximal and        distal walls, an upper bearing surface adapted for gripping an        upper vertebral endplate, and a lower bearing surface adapted        for gripping a lower vertebral endplate, the upper bearing        surface having at least one upper opening therethrough adapted        to promote bony fusion, the lower bearing surface having at        least one lower opening therethrough adapted to promote bony        fusion,    -   c) a first guide surface canal formed in the upper bearing        surface and extending substantially from the proximal wall to        the distal wall, and    -   d) a first rail adapted for slidable reception in the first        canal and having an outward opening recess adapted for reception        of a first screw head.

Also in accordance with the present invention, there is provided aspinal assembly comprising:

-   -   i) a first bone anchor comprising:        -   a) a distal shaft, and        -   b) a proximal screw head, and    -   ii) an intervertebral fusion cage comprising:        -   a) a proximal wall and a distal wall,        -   b) first and second side walls connecting the proximal and            distal walls, an upper bearing surface adapted for gripping            an upper vertebral endplate and a lower bearing surface            adapted for gripping a lower vertebral endplate, the upper            bearing surface having at least one upper opening            therethrough adapted to promote bony fusion, the lower            bearing surface having at least one lower opening            therethrough adapted to promote bony fusion,        -   c) a first guide surface canal formed in the upper bearing            surface and extending substantially from the proximal wall            to the distal wall, the canal adapted for distal reception            of the proximal screw head,            wherein the first screw head is received in the canal.

DESCRIPTION OF THE FIGURES

FIG. 1 discloses a first perspective view of a device of the presentinvention having a single screw and a single guide surface depression oneach of the upper and lower bearing surfaces.

FIG. 2 discloses a second perspective view of the device of FIG. 1.

FIG. 3 discloses a device substantially similar to FIG. 1, but withteeth lining bearing surfaces bordering the guide surface depression.

FIG. 4 discloses a device substantially similar to FIG. 1, but with adiscontinuous guide surface depression.

FIG. 5 discloses a device substantially similar to FIG. 4, but in alateral cage configuration.

FIG. 6 discloses a first device substantially similar to FIG. 1, butwith a cervical cage configuration.

FIG. 7 discloses a second device substantially similar to FIG. 1, butwith a cervical cage configuration.

FIG. 8 discloses a first perspective view of a device substantiallysimilar to FIG. 1, but with a pair of screws inserted into the proximalwall of the device and extending through the respective upper and lowerbearing surfaces.

FIG. 9 discloses a side view of the cage of FIG. 8.

FIG. 10 discloses a first perspective view of a device substantiallysimilar to the cage of FIG. 8, but with a continuous guide surfacedepression.

FIG. 11 discloses a first perspective view of the device of FIG. 10.

FIG. 12 discloses the assembly comprising the components of FIGS. 13 and14.

FIG. 13 discloses a second component of the rail-based device of thepresent invention.

FIG. 14 discloses a first component of the rail-based device of thepresent invention.

FIGS. 15a-15b disclose another embodiment of the present invention.

FIG. 16 shows the device of the present invention being implanted into adisc space with a fusion cage inserter.

FIGS. 17a and 17b disclose a trial of the present invention.

FIGS. 18a-18d disclose various views of the inserter of the presentinvention.

DETAILED DESCRIPTION OF THE INVENTION

Now referring to FIGS. 1 and 2, there is provided an intervertebralfusion cage comprising:

-   -   a) a proximal wall 1 and a distal wall 3,        -   b) first 5 and second 7 side walls connecting the proximal            and distal walls, an upper bearing surface 9 adapted for            gripping an upper vertebral endplate and a lower bearing            surface 11 adapted for gripping a lower vertebral endplate,            the upper bearing surface having at least one upper opening            13 therethrough adapted to promote bony fusion, the lower            bearing surface having at least one lower opening (not            shown) therethrough adapted to promote bony fusion,    -   c) a first guide surface depression 15 formed in the upper        bearing surface and extending substantially from the proximal        wall to the distal wall, the depression adapted for distal        reception of a first screw head 17, and    -   d) a second guide surface depression 19 formed in the lower        bearing surface and extending substantially from the proximal        wall to the distal wall, the second depression adapted for        distal reception of a second screw head.

Typically, the guide surface depression forms a longitudinal canal ineach bearing surface. The distal portion 21 of the guide surfacedepression acts as a means for guiding the more proximal portion of thecanal to the screw head. When the proximal portion 23 of the canal isslid over the screw head, it envelops the screw head, thereby lockingthe cage in place. Further tightening of the screw can be performed tofurther lock the cage in place.

In some embodiments, the cross-sectional profile of the depression orcanal is substantially equivalent to the cross-sectional profile of thescrew head, so that the first depression is well adapted fordistal-to-proximal translation of the first canal towards the screwhead. In some preferred embodiments thereof, the screw head issubstantially spherical, while the transverse cross-section of the firstcanal substantially forms a portion of a circle, thereby providing asubstantially matching fit of the canal and screw head.

In some embodiments, the first canal extends substantially along acenterline of the cage, thereby allowing the use of a single screw perbearing surface.

In some preferred embodiments, the first canal comprises a distal recess25 and a proximal process 27. The proximal process effectively acts tolock the cage in place when it slides over and envelops the screw head.

In some embodiments, the first canal includes an outwardly extending(longitudinal) bump (not shown) adapted to limit translational movementof the interbody device with respect to the screw. This bump acts as anadditional means for guiding the deeper portion of the canal to thescrew head, at which the cage becomes locked.

Now referring to FIGS. 2 and 3, in some embodiments, additional strengthis provided to the proximal portion 29 of the cage in order to withstandhigher tensile forces and insertion forces. Preferably, this additionalstrength is achieved by using a stronger material in the proximalportion of the cage. In preferred embodiments thereof, the bearingsurface 33 surrounding the proximal portion of the canal is formed froma metallic material.

In some embodiments, it is helpful to provide a final seating of thescrew head once it becomes seated in the deeper proximal portion of thecanal. In these embodiments, the proximal wall of the cage preferablyhas a vertical slot 35 that communicates with the horizontal guidesurface canal and is adapted to allow access by a screwdriver. Thus, thesurgeon has direct access to the screw head via a proximal route and caneasily accomplish its final tightening.

In some embodiments (as in FIGS. 1-3), the guide surface canals arecontinuous longitudinal structures that guide the entry of the cage fromthe moment the canal contacts the screw head to the moment the screwseats in the proximal portion of the canal. However, in otherembodiments (as in FIG. 4), the canal 45 a and 45 b may bediscontinuous.

It is believed that the device of the present invention can beadvantageous used in implanting lateral cages. Therefore, now referringto FIG. 5, there is provided a lateral cage 49 of the present invention,wherein the length L of the lateral cage (distal end 47 to proximal end48 distance) is at least two times greater than the width W of thelateral cage (side wall 49 to side wall 50 length). Preferably, thelength of the lateral cage is at least three times greater than thewidth of the lateral cage. Also in FIG. 5, the proximal portion 101 ofthe lateral cage is preferably made of a metal material such astitanium, while the distal portion 103 is made of a polymer-basedmaterial, such as CFRP.

It is believed that the device of the present invention can beadvantageous used in implanting cervical cages. Therefore, now referringto FIGS. 6 and 7, there is provided a cervical cage of the presentinvention having a screw 51 extending from each guide surface depression53.

In some embodiments, it is advantageous to add additional screws to thedevice in order to more completely secure the device to the vertebralendplates. Now referring to FIGS. 8 and 9, there is provided a firstperspective view of a device having a pair of screws 55 inserted intothe proximal wall 57 of the device and extending through the openings inthe respective upper 59 and lower 61 bearing surfaces.

FIG. 10 discloses a distal perspective view of a device substantiallysimilar to the cage of FIG. 8, but with a continuous guide surfacedepression 63. FIG. 11 discloses a proximal perspective view of thedevice of FIG. 10.

Various aspects of the present invention include an implant/instrumentsystem, and a method of implantation. The present invention alsoincludes a kit comprising:

-   -   a) trial instruments comprising interbody spacing blocks having        various sizes (height, angle, footprint), each with bone anchor        placement guides.    -   b) at least two bone fixation anchors to be placed into adjacent        vertebral bodies while trialing with the aforementioned        instrument, and    -   c) an interbody implant configured for engaging with the heads        of the implanted bone anchors after removing the trial        instrument.

Now referring to FIGS. 17a and 17 b, trial 101 has a handle attached toone side (not shown). Two pins are for the trial proper depth position.The vertebral body holes for anchoring the screws can be drilled throughthe guides.

The kit of the present invention allows the surgeon to fix the opposingvertebral bodies to one another through the interbody device withouthaving the implant protrude outside of the disc space. Preferably, theheads of the bone anchors snap into proximal processes formed in canalslocated in the upper and lower surfaces of the interbody implant,thereby helping the implant resist migration. Preferably, the anchorscan be inserted at various angles to accommodate anatomical differencesas well as avoid any pre-existing hardware.

Preferably, the canals of the interbody implant sufficiently envelop therespective bone anchor heads so as to prevent back-out and pull-out ofthe anchor. In such situations, a secondary locking step/feature is notrequired. Preferably, the interbody implant allows passage therethroughof a driver to further seat and tighten the bone anchor into the boneafter the implant has been placed. This is typically accomplished by avertical slot 35 in the proximal wall that communicates with the canals.Preferably, the major diameter of the bone anchor is larger than thescrew head diameter. In some embodiments, the major diameter is 5.5 mm).Larger major screw diameters can be used, as compared to conventionaldevices wherein the anchors are placed through the wall of the fixationdevice and limited by the height of the device. This is a majoradvantage.

The embodiments described herein are preferably designed for thecervical region, but also could be utilized for lumbar spine interbodyfusion as well.

In a preferred embodiment of the invention shown in FIGS. 1 and 2, thebone anchor comprises a screw having a proximal head 17 that issubstantially spherical such that it can accommodate variable angles andstill engage with the interbody implant. Alternatively, other screw headshapes that match the shape of the canal recess and provide a reliableconnection between two components may be used. Also, in preferredembodiments, the screws are positioned in the centerline so that theyare less likely to interfere with an adjacent level plate with twoscrews in each vertebra, as some plates have screws positioned laterallyso the screws of the present invention can go in between them. In use,the screws are first installed and then the interbody implant is slidbetween the vertebrae, using the insertion tool that aligns the centralcanals of the cage with the screw heads. In preferred embodiments, asmall radial bump (not shown) located in the canal limits translationalmovement of the interbody device with respect to the screw. In anotherpreferred embodiment, the screws are positioned such that upon slidingthe interbody device into the disc space, the interbody device slidesprimarily over the screw heads (instead of primarily contacting theendplates—the screws are inserted slightly proud to allow largerdistraction) to ease insertion. At the end of insertion, the screw heads“drop” into a larger proximal process of the canals, thereby ensuringthat the upper and lower surfaces of the interbody device abut the bonyendplates and providing the means of preventing the interbody devicefrom backing out. The interbody device preferably has internal spacesopening outward through the upper and lower bearing surfaces and ontothe adjacent bony endplates. These spaces may be packed with bone graftor bone graft substitute prior to implantation in order to promote bonyfusion of the opposing endplates.

In some embodiments, the proximal wall of the cage has a small verticalslot 35 that provides access by a screwdriver shaft to the guide surfacecanals (and thereby the screw heads). After interbody device insertioninto the disc space, the screws are preferably tightened through theseslots to ensure construct stability. Screw backout is prevented bydesign of the cage and method of cage installation, as the screw headsare seated on the inner surfaces of the respective canals.

FIGS. 12-14 show another embodiment of this invention. In thisembodiment, each screw 201 is inserted into its respective endplatesalong with an additional rail component 203. The rail component has across-section configured to slidingly engage a mating canal in the maininterbody device. In this exemplary embodiment, both the rail and theguide surface canal 205 of the main interbody device 207 have dovetailengagement features, although other reliable locking configurations maybe used. The advantage of the rail lies in its ability to accommodatevariable screw trajectories without the need for a spherical head toensure alignment with the interbody device. Generally, the rail must beimplanted first to envelop the screws, with the screws partiallytightened. Then, after sliding the interbody device over the rails, thescrews are finally tightened, thereby making the whole construct rigidand properly attached to the vertebrae. The cage may be locked to therails by the small springy or bump-like feature (not shown) incorporatedinto the rail and cage design. Preferably, the head of the screw seatson the inner wall of the dovetail canal, thereby preventing the screwfrom backing out.

Revision surgery can be performed by loosening the screws and removingthe cage. In effect, the screws do not need to be removed during cagerevision.

FIGS. 15a and 15b show another embodiment with a pair of screws 221placed to either side of cage midline. The cage is implanted by slidingthe screw heads into the canals of the cage. The final tightening of thescrews may be accomplished through the holes 223 provided on the cage'sproximal wall after the cage is put in place. The holes have a smallerdiameter than the screw head and provide access for a screwdriver shaftwhile preventing screw backout. The cage is locked in place by thescrews' tension, though an additional feature in the form of a smallbump (not shown) can be added to prevent cage expulsion. An injectablebone graft or substitute can be injected through the cage side windowafter final cage assembly. Alternatively, a bone graft substitute suchas TCP may be incorporated into the graft at the situs of manufacture.

FIG. 16 shows the device of the present invention being implanted into adisc space with a fusion cage inserter. The screws 225 are held in placeon tynes distal of the device of the present invention, and are insertedinto the vertebral endplate portion of the disc space so thatsubstantially only their heads protrude from those endplates. Next, thedevice 227 is slid over the screw heads and is locked in place.

Now referring to FIGS. 18a -18 d, a preferred inserter for inserting thepresent invention comprises a body 103, two blades 105 attached to thebody, a pusher 107 attached to the shaft 109 and a handle 111 attachedto the shaft.

To implant the cage, the surgeon has to spread the blades and insert thecage in between the blades, aligning the cage's proximal opening withthe pusher pin 112. The cage has to be positioned along the inserter sothat the pedals are reasonably collapsed in order to be inserted intointervertebral space. The central slots 113 of the blades need to bealigned with the already-implanted screw heads, and the inserter needsto be as vertical as possible (i.e., perpendicular to the anterior planeof the vertebrae) and inserted as deep as the blades' stop surfaces 114will allow. At this point, the inserter handle is turned clockwise,thereby pushing the pusher and the cage forward. During insertion,pedals 115 become distracted, thereby making space for the cage. Thecage is pushed into the disc space until the pusher “stops” contactingthe vertebrae. The cage stops advancing forward and the blades withdrawfrom the disc space by continuing advance of the pusher until the bladesare completely withdrawn.

The pusher blade 119 rides inside the pusher guiding slot 117, therebypreventing the pusher from spinning and aligning it properly to theblades. An impactor (not shown) is then used to advance the cage intothe final position. At this point, the screw heads serve as stops forthe impactor, and the cage cannot move any further distally. At thispoint, the anterior surface of the cage is flush with the screw heads'most protruding points. The final step is the tightening of the screws.

In some embodiments of the present invention, trialing occurs beforeimplantation of the fusion cage. In particular, in accordance with thepresent invention, there is provided a method of inserting a fusion cageinto an intervertebral disc space formed by upper and lower vertebralendplates, comprising the sequential steps of:

-   -   a) inserting a trial having a guide surface canal into the disc        space,    -   b) drilling or awling the hole through the trial drill guides,    -   c) inserting a first and the second screws into the upper and        lower vertebral endplates, the first screw having a shaft and a        screw head,    -   d) removing the trial,    -   e) inserting a fusion cage having a guide surface canal into the        disc space so that the screw head is received in a distal        portion of the canal, and    -   f) distally translating the cage into the disc space so that the        screw head becomes received in a proximal portion of the cage        canal.

These cages of the present invention may be made from any non-resorbablematerial appropriate for human surgical implantation, including but notlimited to, surgically appropriate metals, and non-metallic materials,such as carbon fiber composites, polymers and ceramics.

The interbody device and bone anchors are preferably made out of PEEK orCFRP or any other suitable material providing adequate strength andradiolucency. However, implantable metals such as titanium or stainlesssteel components may be required to ensure adequate strength for eitherthe interbody device or bone anchors. In some cases the interbody devicecan be made as a combination of PEEK and metal. The metal component ispreferably used for screw head retaining feature. In some cases,resorbable materials such as polylactide, polyglycolide, and magnesiumare preferred.

In some embodiments, the cage material is selected from the groupconsisting of PEEK, ceramic and metallic. The cage material ispreferably selected from the group consisting of metal and composite(such as PEEK/carbon fiber).

If a metal is chosen as the material of construction for a component,then the metal is preferably selected from the group consisting oftitanium, titanium alloys (such as Ti-6Al-4V), chrome alloys (such asCrCo or Cr—Co—Mo) and stainless steel.

If a polymer is chosen as a material of construction for a component,then the polymer is preferably selected from the group consisting ofpolyesters, (particularly aromatic esters such as polyalkyleneterephthalates, polyamides; polyalkenes; poly(vinyl fluoride); PTFE;polyarylethyl ketone PAEK; polyphenylene and mixtures thereof.

If a ceramic is chosen as the material of construction for a component,then the ceramic is preferably selected from the group consisting ofalumina, zirconia and mixtures thereof. It is preferred to select analumina-zirconia ceramic, such as BIOLOX delta™, available from CeramTecof Plochingen, Germany. Depending on the material chosen, a smoothsurface coating may be provided thereon to improve performance andreduce particulate wear debris.

In some embodiments, the cage member comprises PEEK. In others, it is aceramic.

In some embodiments, the first component consists essentially of ametallic material, preferably a titanium alloy or a chrome-cobalt alloy.In some embodiments, the second component consists essentially of thesame metallic material as the first plate.

In some embodiments, the components are made of a stainless steel alloy,preferably BioDur® CCM Plus® Alloy available from Carpenter SpecialtyAlloys, Carpenter Technology Corporation of Wyomissing, Pa. In someembodiments, the outer surfaces of the components are coated with asintered beadcoating, preferably Porocoat™, available from DePuyOrthopaedics of Warsaw, Ind.

In some embodiments, the components are made from a composite comprisingcarbon fiber. Composites comprising carbon fiber are advantageous inthat they typically have a strength and stiffness that is superior toneat polymer materials such as a polyarylethyl ketone PAEK. In someembodiments, each component is made from a polymer composite such as aPEKK-carbon fiber composite.

Preferably, the composite comprising carbon fiber further comprises apolymer. Preferably, the polymer is a polyarylethyl ketone (PAEK). Morepreferably, the PAEK is selected from the group consisting ofpolyetherether ketone (PEEK), polyether ketone ketone (PEKK) andpolyether ketone (PEK). In preferred embodiments, the PAEK is PEEK.

In some embodiments, the carbon fiber comprises between 1 vol % and 60vol % (more preferably, between 10 vol % and 50 vol %) of the composite.In some embodiments, the polymer and carbon fibers are homogeneouslymixed. In others, the material is a laminate. In some embodiments, thecarbon fiber is present in a chopped state. Preferably, the choppedcarbon fibers have a median length of between 1 mm and 12 mm, morepreferably between 4.5 mm and 7.5 mm. In some embodiments, the carbonfiber is present as continuous strands.

In especially preferred embodiments, the composite comprises:

-   -   a) 40-99% (more preferably, 60-80 vol %) polyarylethyl ketone        (PAEK), and    -   b) 1-60% (more preferably, 20-40 vol %) carbon fiber, wherein        the polyarylethyl ketone (PAEK) is selected from the group        consisting of polyetherether ketone (PEEK), polyether ketone        ketone (PEKK) and polyether ketone (PEK).

In some embodiments, the composite consists essentially of PAEK andcarbon fiber. More preferably, the composite comprises 60-80 wt % PAEKand 20-40 wt % carbon fiber. Still more preferably the compositecomprises 65-75 wt % PAEK and 25-35 wt % carbon fiber.

Although the present invention has been described with reference to itspreferred embodiments, those skillful in the art will recognize changesthat may be made in form and structure which do not depart from thespirit of the invention.

Alternatively, combinations of cage materials could be beneficial(i.e.,—a ceramic bottom half with a PEEK top half).

In other embodiments, the components are made from resorbable materials,such as Biocryl Rapide™, a PLA, PLG, TCP composite marketed by DePuyMitek, located in Raynham, Mass.

When resorbable materials are selected, Preferred bioresorbablematerials which can be used to make the sutures of the present inventioninclude bioresorbable polymers or copolymers, preferably selected fromthe group consisting of hydroxy acids, (particularly lactic acids andglycolic acids; caprolactone; hydroxybutyrate; dioxanone; orthoesters;orthocarbonates; and aminocarbonates). Preferred bioresorbable materialsalso include natural materials such as chitosan, collagen, cellulose,fibrin, hyaluronic acid; fibronectin, and mixtures thereof. However,synthetic bioresorbable materials are preferred because they can bemanufactured under process specifications which insure repeatableproperties.

A variety of bioabsorbable polymers can be used to make the suture ofthe present invention. Examples of suitable biocompatible, bioabsorbablepolymers include but are not limited to polymers selected from the groupconsisting of aliphatic polyesters, poly(amino acids),copoly(ether-esters), polyalkylenes oxalates, polyamides, tyrosinederived polycarbonates, poly(iminocarbonates), polyorthoesters,polyoxaesters, polyamidoesters, polyoxaesters containing amine groups,poly(anhydrides), polyphosphazenes, biomolecules (i.e., biopolymers suchas collagen, elastin, bioabsorbable starches, etc.) and blends thereof.For the purpose of this invention aliphatic polyesters include, but arenot limited to, homopolymers and copolymers of lactide (which includeslactic acid, D-,L-and meso lactide), glycolide (including glycolicacid), ε-caprolactone, p-dioxanone(1,4-dioxan-2-one), trimethylenecarbonate(1,3-dioxan-2-one), alkyl derivatives of trimethylenecarbonate, δ-valerolactone, β-butyrolactone, χ-butyrolactone,ε-decalactone, hydroxybutyrate, hydroxyvalerate, 1,4-dioxepan-2-one(including its dimer 1,5,8,12-tetraoxacyclotetradecane-7,14-dione),1,5-dioxepan-2-one, 6,6-dimethyl-1,4-dioxan-2-one, 2,5-diketomorpholine,pivalolactone, χ,χ-diethylpropiolactone, ethylene carbonate, ethyleneoxalate, 3-methyl-1,4-dioxane-2,5-dione,3,3-diethyl-1,4-dioxan-2,5-dione, 6,8-dioxabicycloctane-7-one andpolymer blends thereof. Poly(iminocarbonates), for the purpose of thisinvention, are understood to include those polymers as described byKemnitzer and Kohn, in the Handbook of Biodegradable Polymers, edited byDomb, et. al., Hardwood Academic Press, pp. 251-272 (1997).Copoly(ether-esters), for the purpose of this invention, are understoodto include those copolyester-ethers as described in the Journal ofBiomaterials Research, Vol. 22, pages 993-1009, 1988 by Cohn and Younes,and in Polymer Preprints (ACS Division of Polymer Chemistry), Vol.30(1), page 498, 1989 by Cohn (e.g. PEO/PLA). Polyalkylene oxalates, forthe purpose of this invention, include those described in U.S. Pat. Nos.4,208,511; 4,141,087; 4,130,639; 4,140,678; 4,105,034; and 4,205,399.Polyphosphazenes, co-, ter- and higher order mixed monomer-basedpolymers made from L-lactide, D,L-lactide, lactic acid, glycolide,glycolic acid, para-dioxanone, trimethylene carbonate and ε-caprolactonesuch as are described by Allcock in The Encyclopedia of Polymer Science,Vol. 13, pages 31-41, Wiley Intersciences, John Wiley & Sons, 1988 andby Vandorpe, et al in the Handbook of Biodegradable Polymers, edited byDomb, et al, Hardwood Academic Press, pp. 161-182 (1997). Polyanhydridesinclude those derived from diacids of the formHOOC—C₆H₄—O—(CH₂)_(m)—O—C₆H₄—COOH, where m is an integer in the range offrom 2 to 8, and copolymers thereof with aliphatic alpha-omega diacidsof up to 12 carbons. Polyoxaesters, polyoxaamides and polyoxaesterscontaining amines and/or amido groups are described in one or more ofthe following U.S. Pat. Nos. 5,464,929; 5,595,751; 5,597,579; 5,607,687;5,618,552; 5,620,698; 5,645,850; 5,648,088; 5,698,213; 5,700,583; and5,859,150. Polyorthoesters such as those described by Heller in Handbookof Biodegradable Polymers, edited by Domb, et al, Hardwood AcademicPress, pp. 99-118 (1997).

Advantages of the present invention include an ability to have a largerscrew diameters. There is no need for a special screw locking mechanism.The screws are locked by the cage design, following a “screw first”concept. There is high reliability in the screw lock. The cage featuresscrew dynamism, so that there is angle flexibility and no bending stress(tension only). The cage allows screw self-adjustment to the cage. Thecage may have centrally aligned screws, so there is less risk to veinsand arteries. The cage is strongly resistant to axial impact duringinsertion. The screws are closer to the anterior edge. Lastly, there isa possibility of reducing spondylothesis anteriorly.

What is claimed is:
 1. A method of inserting a fusion cage into anintervertebral disc space formed by upper and lower vertebral endplates,comprising the sequential steps of: inserting a trial having a guidesurface canal into the disc space; drilling or awling a hole through atrial drill guide; inserting first and second screws into the upper andlower vertebral endplates, the first screw having a shaft and a screwhead; removing the trial; inserting a fusion cage having a guide surfacecanal into the disc space so that the screw head is received in a distalportion of the canal; and distally translating the cage into the discspace so that the screw head becomes received in a proximal portion ofthe cage canal.